The present invention relates generally to an anti-skid brake control system and, more particularly, to such a system that detects changes in the speed of the more slowly rotating one of a plurality of vehicle wheels monitored by the system.
Anti-skid brake control systems are shown in U.S. Pat. No. 3,917,359 and in co-pending U.S. patent applications Ser. No. 685,267, filed May 11, 1976, for "Anti-Skid Brake Control System With Short Circuit Protection" now U.S. Pat. No. 4,040,676, issued Aug. 9, 1977, and Ser. No. 769,255, filed Feb. 16, 1977, for "Anti-Skid Brake control System With Power-Up Delay". Both of the mentioned applications are assigned to the same assignee as the present application.
It is the principal purpose of an anti-skid brake control system to provide automatic overriding control of the brakes of the vehicle when an incipient or actual skid or a locked wheel condition exists. Upon detecting such condition, the system dumps part or all (hereinafter a percentage) of the brake operating fluid pressure (hereinafter air pressure) which the vehicle operator then may be attempting to apply manually by pressing on the vehicle brake pedal with his foot, for example.
In such a system a transducer detects the rotational speed of a vehicle wheel and produces an AC transducer signal having a frequency representative of the wheel speed. A frequency to voltage converter produces a DC voltage that has an amplitude indicative of the frequency of the AC transducer signal and, thus, representative of the wheel speed.
A deceleration detector monitors downward-going changes in the DC voltage indicative of wheel deceleration and produces a deceleration signal representative of the magnitude of such deceleration. If the deceleration exceeds a predetermined threshold amount, which would be indicative of an incipient or actual skid condition of the wheel, the system dumps a percentage of the air pressure. Moreover, in case the wheel locks up, for example, whereupon there would be no further deceleration thereof, a wheel lock detector detects the same and continues the pressure dump until the wheel spins up again by at least a minimum amount. However, it has been found undesirable to re-apply full braking force to a relatively rapidly moving vehicle promptly after a wheel locked condition has terminated, for the wheel may too easily lock again; therefore, an acceleration detector monitors upward-going changes in the DC voltage indicative of wheel acceleration and produces an acceleration signal which if over a threshold level causes dumping of some, less than all, of the air pressure then being called for by the driver.
When a pair of wheels on a common axle but on opposite sides of the vehicle are to be monitored by an anti-skid brake control system as above, a separate transducer is coupled relative to each wheel to produce a pair of AC transducer signals having respective frequencies indicative of the respective wheel speeds. A pair of frequency to voltage converters convert those AC signals to respective generally DC voltages. Moreover, in the past a pair of deceleration differentiating circuits have been coupled to monitor the respective DC voltages and to produce respective deceleration signals representative of the deceleration of the respective wheels, and the two deceleration signals were combined, for example, in an OR gate and delivered to a control circuit. If either of the deceleration signals exceeded a predetermined threshold value, then the control circuit would produce an output signal effecting dumping of a percentage of the air pressure being called for by the vehicle driver. Similarly, a pair of acceleration differentiating circuits have been coupled to the respective frequency to voltage converters to produce respective acceleration signals indicative of wheel acceleration or spin-up, and the acceleration signals were combined in another OR gate and delivered to the control circuit to effect a pressure dump if either acceleration signal exceeded a predetermined amount. Accordingly, for each monitored wheel it was necessary to have a transducer, a frequency to voltage converter, a deceleration differentiating circuit, and an acceleration differentiating circuit, and the respective deceleration and acceleration signals produced by these components were delivered to a common control circuit.
In U.S. Pat. No. 3,847,446 there is disclosed a skid control system for tandem axled vehicles. Such system is intended to control the brakes of adjacent tandem wheels, i.e. those on the same side of the vehicle on two closely adjacent axles. Several embodiments are disclosed in such patent including one in which an incipient skid condition must be detected at both wheels before air pressure can be dumped and another in which the pressure dump is at least partially disabled when the first of the two wheels spins up. However, in all of the embodiments disclosed in such patent for each wheel monitored there is required a sensor mechanism that produces a voltage indicative of the wheel speed, a deceleration differentiating circuit, and an acceleration differentiating circuit, and there is a common control circuit, then, that effects a pressure dump when necessary.